Out of the 195 nations and entities recognised by the UN, Holtorf and his 1988 Mercedes-Benz Gelandewagen — a 300GD, if you're absolutely dying to know — have visited 179. In the process Holtorf and the G-Wagen, nicknamed Otto, have covered an amazing 884,000km on the road, which is more than double the distance between the Earth and the Moon.
Around 1990 Holtorf fitted a large bed in back of Otto, underneath which there was a large storage space for supplies. On the roof Holtorf installed a series of aluminium containers to store around 400 spare parts.
Given the nature of the many of the roads traversed, running repairs were a fact of life, but Otto hasn't required any major rebuilds. According to the BBC's comprehensive account of his trip, titled "Gunther, Christine and Otto", Holtorf took many precautions, including never going over 80km/h, deflating Otto's tyres slightly before driving on rutted tracks and filtering any suspicious looking fuel.
In Holtorf's opinion, thanks to the rise of automotive electronics, it would be impossible recreate or better Otto's round-the-world trip with a modern vehicle. "Otto is nothing but nuts and bolts, that means ... [I can] repair anything that comes up myself. In any modern car you cannot touch the brakes because it's all electronically controlled," he told the British broadcaster.
The majority of Otto's journey was done in the company of Gunther and his fourth wife Christine. Until 2003, when Christine was diagnosed with cancer, Otto propelled Gunther and Christine around most of Africa and all of the Americas. In the subsequent years Otto, Gunther, occasionally Christine before her death in 2010 and other companions trekked across Africa, Asia, Europe and Australia.
For much of the voyage, Otto and the Holtorfs only travelled for a few months of the year. During that time they slept in the open or in the back of the G-Wagen, and actively avoided large cities and hotels unless absolutely necessary.
Otto's journey around the world was funded by Holtorf's thirty years of service to Lufthansa, as well as the proceeds from the 450-page Jakarta-Jabotabek Street Atlas and Names Index that he began compiling during his time in the Indonesian capital in the 70s.
Although there were plenty of companies willing to sponsor Otto, Holtorf resisted the temptation of their lucre. Not only did Holtorf wish to avoid the obligations that sponsors would ultimately demand, but he believes steadfastly that travelling unobtrusively made his travels immeasurably safer.
He did, however, on occasion accept a helping hand from Mercedes-Benz. The manufacturer not only helped to fill out the reams upon reams of paperwork required to traverse China, but also convinced AXA to provide Otto with a unique worldwide third-party insurance scheme.
For some countries entering via car is definitely not the done thing. For example, some Caribbean countries had to convene cabinet to discuss whether to let Otto and his crew onto their islands. All of that paled in comparison, though, to the high-level diplomatic contact required between Germany and North Korea for Holtorf to strike the hermit kingdom off his list.
Asked by the BBC to name the friendliest countries he's been to during his travels, Gunther nominated Ghana, before adding Australia to the list. Indeed, were it not for our stringent quarantine rules that required an expensive and time consuming sterilisation process, Australia would have been "perfect".
Holtorf estimates that over the years, including fuel, repairs, spares and transport, he's spent 450,000 euros ($653,000) on the globe trekking G-Wagen. Much of that, he hopes, will be recouped when the Mercedes-Benz Museum in Stuttgart purchases Otto.